Gravure is a major commercial printing processes that can be used to print, on a substrate such as paper, anything from newspaper to fine art. Text and images can be printed.
Gravure is an intaglio process wherein ink is transferred to the paper as drops from very small cells that are recessed into a printing surface, e.g., a cylinder or flat plate. The ink drops flow and selectively spread together to print the text or image. If the surface tension of the ink drop is to high, the ink will not spread quick or far enough causing the print to appear rough and grainy.
Gravure is distinguished from other processes such as letterpress printing and lithography. In letterpress printing, ink is transferred to the paper from a raised image. In lithography, the entire printing surface has areas that will print defined by a water repellant ink and the areas that will not print coated with a film of water to prevent the ink from contacting the surface. Each of these printing processes require distinct inks having physical properties tailored for the specific process.
Gravure inks are very fluid, solvent or co-solvent based inks that dry by evaporation to leave a film of resin and pigment on the substrate. Representative solvents and co-solvents include toluene, xylene, alcohols, acetone, aliphatic hydrocarbons, water and the like. Except for water, the solvents and co-solvents are volatile organic compounds (VOC). Due to environmental considerations, limitations are being placed on the amount of VOC emissions that occur during printing. Solvent recovery systems or incinerators are utilized to reduce the VOC emissions but treat the effect rather than the cause of the problem.
Waterborne inks that includes as solvents water and a co-solvent, e.g., alcohols, emit less VOC than solvent based inks. The co-solvent improves the printability of waterborne inks by reducing the surface tension. The relatively large amount of co-solvent required to reduce the surface tension result in a waterborne ink having a very high VOC content.
True waterbased gravure inks, i.e., inks that only use water as a solvent and do not include a co-solvent, provide poor print quality. The printed paper is roughened because the ink wets and swells the paper fibers. The print is rough and grainy because of the roughening and also because the chemical formulation of the ink results in poor ink transfer to the paper, poor ink dot spreading on the paper and poor trapping, i.e., over printing, of one ink on another. Thus, the printability is poor.
Waterbased gravure inks can also include volatile surfactants and defoamers. Unfortunately these surfactants and defoamers are inefficient and require relatively large dosages, e.g., 2 weight percent. (wt %)per component, which adds to the VOC content of the ink.
A waterbased gravure ink having a low VOC content and that provides good print quality is desirable.